1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
use_prelude!();

use ::core::mem::ManuallyDrop;

#[cfg(doc)]
use crate::extension_traits::ManuallyDropMut;

/// Extension trait to convert a `&mut _` into a `&out _` by calling
/// `.as_out()` on it.
///
/// By autoref, this means that you can even just extract a `&out T` reference
/// out of a `mut` element simply by calling `.as_out()` on it.
///
/// There is, however, one restriction: to be able to call `.as_out()` on
/// something, it needs to be either `Copy`, or a value wrapped in a
/// [`MaybeUninit`] / a [`ManuallyDrop`].
///
/// This is by design. Indeed, [`Out`] references do not call the destructor
/// of the overwritten element (since it may not be initialized).
/// This could cause memory leaks when there is an initialized element with
/// [drop glue][`core::mem::needs_drop`].
///
/// To solve this limitation, one must explicitly call
/// [`.manually_drop_mut()`][`ManuallyDropMut::manually_drop_mut`]
/// to automagically transmute the `&mut _` reference into a
/// `&mut ManuallyDrop<_>`.
///
/// # Examples
///
/// ```rust
/// use ::uninit::prelude::*;
///
/// let mut x = 0;
/// x.as_out().write(42);
///
/// let mut y = ::core::mem::MaybeUninit::uninit();
/// y.as_out().write(42);
/// let y = unsafe { y.assume_init() };
///
/// assert_eq!(x, y);
/// ```
pub
trait AsOut<Pointee : ?Sized> {
    #[allow(missing_docs)]
    fn as_out<'out> (self: &'out mut Self)
      -> Out<'out, Pointee>
    ;
}

impl<T> AsOut<T> for MaybeUninit<T> {
    #[inline]
    fn as_out<'out> (self: &'out mut MaybeUninit<T>)
      -> Out<'out, T>
    {
        self.into()
    }
}

impl<T> AsOut<T> for T
where
    T : Copy,
{
    #[inline]
    fn as_out<'out> (self: &'out mut T)
      -> Out<'out, T>
    {
        self.into()
    }
}

impl<T> AsOut<[T]> for [MaybeUninit<T>] {
    #[inline]
    fn as_out<'out> (self: &'out mut [MaybeUninit<T>])
      -> Out<'out, [T]>
    {
        self.into()
    }
}

impl<T> AsOut<[T]> for [T]
where
    T : Copy,
{
    #[inline]
    fn as_out<'out> (self: &'out mut [T])
      -> Out<'out, [T]>
    {
        self.into()
    }
}

impl<T> AsOut<T> for ManuallyDrop<T> {
    #[inline]
    fn as_out<'out> (self: &'out mut ManuallyDrop<T>)
      -> Out<'out, T>
    {
        self.into()
    }
}

impl<T> AsOut<[T]> for [ManuallyDrop<T>] {
    #[inline]
    fn as_out<'out> (self: &'out mut [ManuallyDrop<T>])
      -> Out<'out, [T]>
    {
        self.into()
    }
}

#[cfg(not(feature = "const_generics"))]
const _: () = {
    macro_rules! impl_arrays {( $($N:tt)* ) => ($(
        impl<T> AsOut<[T]> for [MaybeUninit<T>; $N] {
            #[inline]
            fn as_out<'out> (self: &'out mut Self)
              -> Out<'out, [T]>
            {
                From::from(&mut self[..])
            }
        }
        impl<T> AsOut<[T]> for [T; $N]
        where
            T : Copy,
        {
            #[inline]
            fn as_out<'out> (self: &'out mut Self)
              -> Out<'out, [T]>
            {
                From::from(&mut self[..])
            }
        }
    )*)}

    impl_arrays! {
           0  1  2  3  4  5  6  7
           8  9 10 11 12 13 14 15
          16 17 18 19 20 21 22 23
          24 25 26 27 28 29 30 31
          32 33 34 35 36 37 38 39
          40 41 42 43 44 45 46 47
          48 49 50 51 52 53 54 55
          56 57 58 59 60 61 62 63
          64
         128
         256
         512
        1024
    }
};

#[cfg(feature = "const_generics")]
const _: () = {
    #[cfg_attr(feature = "better-docs",
        doc(cfg(feature = "const_generics")),
    )]
    impl<T, const N: usize> AsOut<[T]> for [MaybeUninit<T>; N] {
        #[inline]
        fn as_out<'out> (self: &'out mut Self)
          -> Out<'out, [T]>
        {
            From::from(&mut self[..])
        }
    }

    #[cfg_attr(feature = "better-docs",
        doc(cfg(feature = "const_generics")),
    )]
    impl<T, const N: usize> AsOut<[T]> for [T; N]
    where
        T : Copy,
    {
        #[inline]
        fn as_out<'out> (self: &'out mut Self)
          -> Out<'out, [T]>
        {
            From::from(&mut self[..])
        }
    }
};